proportion

A statement that two ratios are equivalent,
written as a:b = c:d, as in the statement
2:3 = 6:9. Two functions are proportional
if for all x, f(x) = kg(x),
where k is the constant of proportionality.
If the value of k is not known, the statement may be written f(x)
g(x). A continued proportion is a group of three or more quantities, each bearing the same ratio to its successor, such as the sequence 1:3:9:27:81.